3D printed active electronic materials and devices

ABSTRACT

Disclosed is a process whereby diverse classes of materials can be 3D printed and fully integrated into device components with active properties. An exemplary embodiment shows the seamless interweaving of five different materials, including (1) emissive semiconducting inorganic nanoparticles, (2) an elastomeric matrix, (3) organic polymers as charge transport layers, (4) solid and liquid metal leads, and (5) a UV-adhesive transparent substrate layer, demonstrating the integrated functionality of these materials. Further disclosed is a device for printing these fully integrated 3D devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application No.62/107,126, filed Jan. 23, 2015, which is hereby incorporated in itsentirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.FA9550-12-1-0368 awarded by the Air Force Office of Scientific Research(AFOSR) and Grant No. D12AP00245 awarded by the Defense AdvancedResearch Projects Agency (DARPA). The government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

Three-dimensional (“3D”) printing is a technique that is starting togain significant attention and commercial interest. However, to date, 3Dprinting has been limited to components such as plastic parts and metallines, as well as to specific plastics, passive conductors, and a fewbiological materials. A significant advance would be the ability to 3Dprint functional active electronic materials and devices in a variety ofgeometries, beyond the two dimensional (“2D”) constraints of traditionalmicrofabrication semiconductor processing. Developing the ability to 3Dprint various classes of materials possessing distinct properties couldenable the freeform generation of active electronics in uniquefunctional, interwoven architectures. Achieving seamless integration ofdiverse materials with 3D printing is a significant challenge thatrequires overcoming discrepancies in material properties in addition toensuring that all the materials are compatible with the 3D printingprocess.

The freeform generation of active electronics in unique architectureswhich transcend the planarity inherent to conventional microfabricationtechniques has been an area of increasing scientific interest.Three-dimensional large-scale integration (3D-LSI) can reduce theoverall footprint and power consumption of electronics, and is usuallyaccomplished via stacks of two dimensional semiconductor wafers, inwhich interconnects between layers are achieved using wire-bonding orthrough-silicon vias. Overcoming this “2D barrier” has significantpotential applications beyond improving the scalability in semiconductorintegration technologies. For instance, the ability to seamlesslyincorporate electronics into three-dimensional constructs could impartfunctionalities to biological and mechanical systems, such as advancedoptical, computation or sensing capabilities. For example, integrationof electronics on otherwise passive structural medical instruments suchas catheters, gloves, and contact lenses are critical for nextgeneration applications such as real-time monitoring of physiologicalconditions. Such integration has been previously demonstrated viameticulous transfer printing of pre-fabricated electronics and/orinterfacing materials via dissolvable media such as silk on nonplanarsurface topologies. An alternative approach is to attempt to interweaveelectronics in three dimensions from the bottom up. Yet, attainingseamless interweaving of electronics is challenging due to the inherentmaterial incompatibilities and geometrical constraints of traditionalmicro-fabrication processing techniques.

BRIEF SUMMARY OF THE INVENTION

Disclosed herein are novel strategies directed at the seamlessinterweaving of three-dimensional active electronic devices.

Also disclosed is a method of making a device involving 3D printing anactive electronic device composed of semiconducting materials. Thismethod may also include 3D printing of elastomeric matrices, organicpolymers, solid and liquid metal leads, nanoparticle semiconductors,and/or transparent substrate layers. This method may also involveidentification of at least one material of an electrode, semiconductor,or polymer that possesses a desirable functionality and that exists in aprintable format, and then patterning that material via directdispensing from a CAD-designed construct onto a substrate. Conformal 3Dprinting, such as printing onto a curved surface like a contact lens,may also require scanning the topology of the surface of the substrate,and providing that information to the CAD system. The substrate can be avariety of desirable materials exhibiting flat or non-flat surface, suchas biologics, glass, polyamides, or 3D printed substrates. Thesedisclosed semiconductors may provide a multitude of end uses, such aswearable displays and/or continuous on-eye glucose sensors. Thesedevices may also include a range of functionality, from includingquantum dot light-emitting diodes (QD-LEDs), MEMS devices, transistors,solar cells, piezoelectrics, batteries, fuel cells, and photodiodes. Thedisclosed method may also incorporate other classes of nanoscalefunctional building blocks and devices, including metallic,semiconductor, plasmonic, biological, and ferroelectric materials. Thedisclosed method may also involve several steps for printing asemiconducting material, including providing a syringe with apredetermined nozzle tip size, loading ink comprising the semiconductingmaterial into the syringe, placing the syringe under vacuum, turning offthe vacuum, lowering the syringe until the ink at the nozzle tip touchesthe substrate, holding the syringe in place for a predetermined periodof time, and raising the syringe and placing the syringe under vacuum.

The disclosed method may also include connecting two active electronicdevices together via a printed conductive pattern.

In producing an active electronic device, the disclosed method may alsoinclude dissolving or suspending a printable material in a compositioncomprising at least one orthogonal solvent, and may involve dissolvingor suspending the printable material at a concentration of less than 1wt %, often lower than 0.20 wt %.

Also disclosed is a method of making a particular active electronicdevice with a 3D printer, a quantum dot light emitting diode (QD-LED),involving printing circular rings connected to contact pads on asubstrate using a conductive nanop article ink, such as silver,annealing the printed conductive nanoparticles, dispensing a conductivepolymer, such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate(PEDOT:PSS), at approximately the center of the printed circular ringuntil the contact line touches the printed circular ring, heating thesubstrate, dispensing and annealing a solution comprising between about0.05 and about 0.20 wt %Poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidine (poly-TPD) inchlorobenzene, dispensing CdSe/ZnS QDs in a co-solvent mixture onto theannealed poly-TPD, drying the QDs, printing a liquid metal, such asEGaIn, at approximately the center of the printed circular ring,printing a UV adhesive around the liquid metal to insulate it from thesilver nanop article (“AgNP”) anode ring, curing the printed adhesivewith a laser, printing conductive silicone, and printing and curingultraviolet (UV) adhesive to encapsulate the printed QD-LED. This methodmay also require printing conductive silicone as vertical interconnectsalong with room temperature vulcanized (RTV) silicone to connect theexposed liquid metal to the contact pads and to connect the anode andcathode of different QD-LED layers so as to 3D print an array ofQD-LEDs. The disclosed method may also involve scanning the surfaceusing a 3D scanner to generate a geometrically faithful computer modelof the surface, providing the computer model to the 3D printer, andadjusting the 3D printing process to enable conformal 3D printing on asubstrate having a non-flat, 3D surface or a flat surface which hasstructural elements on the surface, for example, a rough flat surface.

Also disclosed is a 3D Printer configured for printing these activeelectronic devices, which requires a plurality of syringes, eachcomprising a barrel and nozzle, a printer stage located under theplurality of syringes, a printer stage heating and cooling unit, atleast one pressure regulator connected via tubing to at least one of theplurality of syringes, and a UV laser.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of one embodiment of a 3D printed device.

FIG. 2 is a diagram illustrating connections between active electronicdevices within a single device.

FIG. 3 is a block diagram of an embodiment of a 3D printer capable ofprinting active electronic devices.

FIG. 4 is a diagram of several steps involved in one embodiment of theprinting process for 3D printing an active electronic device.

FIG. 5 is a diagram of an array of active electronic devices.

DETAILED DESCRIPTION OF THE INVENTION

The novel methods and systems described herein are aimed at enabling theseamless interweaving of electronics in three dimensional devices,generally built from the bottom up.

The present invention discloses a method of making a device, whichcomprises 3D printing an active electronic device. These devicesgenerally contain semiconducting materials, which include but are notlimited to silicon, silicon-based compounds, germanium, germanium basedcompounds, cadmium-based compounds, and gallium-based compounds. Thoseskilled in the art will recognize that this list is non-exhaustive.

The present invention also may require 3D printing of additionalmaterials or layers beyond a semiconductive material. Those layers mayinclude an elastomeric matrix, organic polymers as charge transportlayers, solid or liquid metal leads, nanoparticle semiconductors, and aUV-adhesive transparent substrate layer. Other classes of nanoscalefunctional building blocks and devices may also be incorporated,including metallic, semiconductor, plasmonic, biological, andferroelectric materials.

As illustrated in FIG. 1, the active electronic device 10 may includemultiple layers. The device 10 may include a connector layer 20, whichmay comprise silver nanoparticles. In FIG. 1, connector layer 20 isshown as a having a ring-shaped configuration. While this configurationmay be appropriate in some instances, other configurations may bedesirable. An adhesive layer 30 may be required to ensure adhesion to asubstrate 12 which may have contact pads 14 on it. An anode layer 40 maythen be printed, which may include materials such as PEDOT:PSS. Thedevice may then have a hole transport layer 50, such as Poly-TPD,printed, followed by a semiconductor layer 60, perhaps an emission layerusing CdSe-ZnS quantum dots. An insulation layer 70 may then berequired. Similar to the connector layer 20, the insulation layer 70 isshown as having a ring-shaped configuration in FIG. 1. Such aconfiguration may not be necessary in all devices, and otherconfigurations may be desirable. A cathode layer 80, such as aluminum oreutectic gallium indium liquid metal, may then be printed, and a finalconnection layer 90 can optionally be included. One skilled in the artwill recognize that this is merely an illustration of one embodiment,and alternate arrangements of layers fall within the scope of thisinvention,

This process often requires identifying at least one material thatpossesses desired functionalities, including conductors, semiconductors,and insulators, and is not restricted to metals or polymers.

Materials for utilization in this process should generally be in aprintable format. That is, they can be printed directly as-is, they canbe modified to allow them to print, or they can be combined with anothermaterial to enable them to print. This can include, but is not limitedto, chemically or physically manipulating a material, heating or coolinga material, dissolving or suspending a material in at least one solvent,surface coating a material, or introducing a processing aid.

As one example, Polyacrylic acid-capped AgNPs were synthesized bymodifying the synthesis method disclosed by Ahn et al. Specifically, theparticles were precipitated via centrifugation at 7,830 rpm for 1 hour(Eppendorf, Hauppauge, N.Y.) and were homogenized at a speed of 100 rpmfor 10 min using an automatic solder paste mixing machine (Japan UnixCo, Akasaka, Japan). The morphology of the printed AgNP wascharacterized by scanning electron microscopy (Quanta 200 FEGEnvironmental SEM). The silver precipitate was characterized with atransmission electron microscope (Philips CM100). The particle sizeswere measured from the TEM image and found to have an average diameterof 8.2±5.0 nm. The printed AgNP had a resistivity of 10 Ω/cm after anhour of heating at 200° C.

As another example, Poly-TPD (American Dye Source, Quebec, Canada) wasdissolved in chlorobenzene (Fisher Scientific, Pittsburgh, Pa.) anddispensed on top of printed PEDOT:PSS. In contrast to spin coating, inwhich a significantly higher concentration (1.5 wt %) is typically usedas the starting solution, a device with appropriate diodecharacteristics was obtained when the concentration was reduced 10-18fold (around 0.0825 wt % to around 0.15 wt %) during direct ink 3Dprinting. Concentrations of 1.5 wt % resulted in non-uniform films withbulk resistances that were too high for this particular application(2.44×10⁹Ω at 10 V). Significantly diluted concentrations of poly-TPD(0.015 wt %) yielded unstable device performances due to thediscontinuous surface of the poly-TPD layer. It should be noted thatalthough this particular combination had a workable range of betweenabout 0.02 wt % and about 0.20 wt %, one skilled in the art willrecognize that the exact maximum and minimum concentrations within whicha given combination of materials and solvents will produce a continuoussurface that is thin or thick enough to meet a particular applicationrequirement will vary depending on the materials and solvents involved.

In one particular embodiment, the material is dissolved or suspended ina composition that comprises an orthogonal solvent. Utilizing orthogonalsolvents minimizes the risk of compromising the integrity of theunderlying layers during the printing process. In another embodiment,the material is dissolved or suspended in a composition that comprisestwo orthogonal solvents. In a preferred embodiment, the material ispresent at between about 0.02 wt % and about 0.20 wt % in a compositioncomprising at least one solvent.

This method can exploit solutal Marangoni effects, which suppress theaccumulation of suspended particles in a drying droplet of solution nearthe pinned contact line that is due to the capillary flow, i.e. theso-called coffee ring pattern. The Marangoni effect is known to be dueto the different surface tensions associated with the two liquidcomponents of a binary mixture, which create a solutal Marangoni flow.Under optimal conditions, the particles may deposit in a self-assembleduniform layer. Thus, by appropriately selecting orthogonal solvents,this method can produce uniform coatings that utilize significantly lessconcentrated solutions of the material than what is seen inspin-coating.

As one example, CdSe/ZnS QDs (Ocean NanoTech, San Diego, Calif.) wasdissolved in toluene (Sigma Aldrich, St. Louis, Mo.) to 10 mg/ml.Dichlorobenzene (Sigma Aldrich, St. Louis, Mo.) and toluene were thenadded to dilute the concentration to 3 mg/ml with a volume fraction of0-50% dichlorobenzene. A 25 mm×25 mm indium tin oxide (ITO) coated glassslide (Sigma Aldrich, St. Louis, Mo.) was cleaned with water, acetoneand isopropanol for 15 minutes each. The ITO was then dried withnitrogen and heated to 150° C. for 15 minutes. A 0.8 wt % PEDOT:PSS(Sigma Aldrich, St. Louis, Mo.) solution was then spin coated and heatedat 150° C. with a hotplate for 15 minutes. A 1.5 wt % poly-TPD (AmericanDye Source, Quebec, Canada) solution was then spin coated and heated at110° C. for 30 minutes. 0.5 μL of prepared QD solution was thendispensed with an auto micropipette. The experiments were conductedunder relative humidity of 22.5±1%, and temperature 23.5±0.5° C. A hoodwas installed on top of the droplet to minimize disturbance from theenvironment while the mixture droplets evaporate. The QD dropletevaporation was visualized with the excitation from a 527 nm fluorescentlamp. The emission was filtered with a 540 nm filter and the fluorescentsignal was recorded using a CCD camera. The lamp power was kept constantthroughout the experiment, so that the intensity captured correlatedwith the deposition concentrations of the QDs of different droplets. Thefilm heights were measured with a surface profiler (KLATencor/P-15). Itcan be seen that for the pure toluene case, the contact line recedes asevaporation occurs. Therefore, most QDs are concentrated in a smallregion near the droplet center and distributed over a significantlysmaller region than the target area. Further characterization byprofilometry at a 1 mm radius shows a maximum height of 5.6 μm at thereceded coffee ring and low uniformity of the QD layer as indicated by aroot-mean-square roughness value (RRMS) of 900 nm at the inner circularregion. This lack of uniformity in the printed QDs may not be desirable,as it can cause significant difficulties in subsequent printing steps,reducing device performance and yield.

A binary mixture of 80% toluene and 20% dichlorobenzene allows thecontact line to remain pinned everywhere, resulting in the formulationof a more uniformly distributed QD layer, except at the outer region ofthe contact line. For example, at the same 1 mm radius region, theprofilometer measurement shows a maximum height of only 260 nm and RRMSof 110 nm. The annular ring pattern deposition observed is due to thestick-and-slip mode of the contact line movement. Further increasing thedichlorobenzene concentration did not improve QD layer uniformity inthis example.

The present invention further discloses how patterns may be created.While these materials may simply be dispensed at a fixed location or byin a pattern that is hard-coded into a unit controlling the printer (asmight exist in a high speed manufacturing facility), the presentinvention also incorporates patterning of at least one identifiedmaterial via direct dispensing from a CAD-designed construct onto asubstrate.

Further, the CAD-designed constructs may be created utilizing designsfor surfaces created entirely within the CAD system, one embodiment ofthis invention also envisions a more accurate method of conformalprinting. This method involves the additional steps of scanning thetopology of the surface of a substrate that is desired to be printedupon, then providing the information derived from the scanning step intothe CAD design of the device. The substrate being printed upon may ormay not have a variety of characteristics, or a combination thereof,including being flat, biological, porous, glass, polymeric, or a 3Dprinted substrate. In a particular embodiment, the surface is a contactlens.

The type of features being printed may include, but is not limited to,quantum dot light-emitting diodes (“QD-LEDs”), MEMS devices,transistors, solar cells, piezoelectrics, batteries, fuel cells, andphotodiodes. In one embodiment, these features may combine to form, forexample, a wearable display, and/or a continuous on-eye glucose sensor.

As one example, a hard contact lens (Winchester Optical Company, Elmira,N.Y.) was imaged using a commercially available 3D structured-lightscanner (SLS-1, David Visions, Germany) which resulted in geometricallyfaithful computer models of the contact lens surface, here representingthe target curvilinear surface for conformal printing of QD-LEDs. Priorto scanning, a thin layer of contrast agent was applied to the lens toincrease the density of data acquired per scan. The lens was thenmounted at the center of a motorized rotational stage (Thorlabs, Newton,N.J.). The scanner was calibrated and focused following vendor-providedprotocols. Raw scan data was acquired without the use of scanningsoftware-associated smoothing or filtering algorithms and was saved inwavefront OBJ data format. A total of eight scans of the lens wereobtained at different rotational positions of the stage, ranging from 0to 360 degrees in 45 degree intervals.

The individual wavefront files were subsequently aligned and assembledusing a mesh editing software (MeshLab) which resulted in a 3D meshreconstruction of the contact lens. Briefly, alignment and assembly werecarried out by point-based gluing alignment of the individual scansusing approximately six identification points per scan. The data wasthen converted to a single 3D mesh by flattening visible layers, fillingholes, and reconstructing a global surface.

The scanned model was then imported to Solidworks Premium 2014 with aScanTo3D feature before the QD-LED CAD model was conformed to thesurface of the model. A layer of UV adhesive (Novacentrix, Austin, Tex.)was then printed on the scanned hard contact lens as an adhesion layer.The adhesion layer was then cured with a handheld UV-lamp (285 nm) foran hour. The QD-LED was then printed, in which a concentration of 3mg/ml solution of orange-red QDs (Ocean NanoTech, San Diego, Calif.) and50% dichlorobenzene was used to achieve electroluminescence on theacrylate-based adhesive. To heat the layers without damaging thesubstrate, the device was heated near the top layers via inversion, witha 1 mm air gap between the surface of the contact lens and the hotplate. The heating time was extended until the AgNP was sintered. Thesubstrate was observed periodically and the gap was adjusted to preventcharring. For other layers, the hot plate temperature of the invertedconfiguration was increased until the temperature of the surface reachedthe prescribed surface temperature.

As another example, a 3D CAD model, including the substrate and QD-LEDarray, were designed and rendered using Solidworks Premium 2014(Dassault Systèmes, Vélizy-Villacoublay Cedex, France), then sent to the3D printer for printing. Contact pads and connectors were printed on aglass substrate with synthesized silver nanoparticles (33 ga, 80 μm gap,50 psi pressure, 0.5 mm/s translation speed). A room temperaturevulcanized silicone sealant (Loctitie, Rocky Hill, Conn.) was printed asthe structural material. For silicone printing, tapered nozzle sizesranged from 20 ga to 25 ga (610 μm to 250 μm) and the parameters weretailored based on the feature size, resolution and print speed. Theprint gap was maintained at 80% of the nozzle inner diameter, and thesyringe barrel pressure ranged from 20-50 psi, depending on thetranslation speed. Prior to the printing of QD-LEDs on the printedsilicone substrate, the UV adhesive (Novacentrix, Austin, Tex.) wasprinted and cured with a UV laser (405 nm) and a handheld UV lamp (285nm). Multicolor QD-LEDs were then printed using the procedures describedabove. To heat the layers without damaging the substrate, the device wasinverted and heated near the top layers, with a ˜1 mm air gap betweenthe surface of the contact lens and the hot plate. The heating time wasextended until the printed AgNPs were sintered. The substrate wasobserved periodically and the gap was adjusted to prevent charring. Forother layers, the hot plate temperature of the inverted configurationwas increased until the temperature of the surface reached theprescribed surface temperature.

One skilled in the art will recognize that the characteristics of thesubstrate and the interaction between the substrate and the device willalso need to be taken into consideration. In some cases, the device maybe printed directly onto the substrate. In others, an adhesive layer maysometimes be required to allow the printing of a device onto a desiredsubstrate.

As one example of this, the suitability of five different printabletransparent polymers as QD-LED substrates was assessed. The contactangle of a PEDOT:PSS ink on (A) Novacentrix, PRO-001 UV, (B) polyvinylalcohol (PVA), (C) polydimethylsiloxane (PDMS) (D) Norland Products Inc,UVS 91, (E) bifunctional acrylate monomers with photoinitiator(polyacrylate), and (F) glass, were investigated. PRO-001 UV consists ofa blend of acrylate monomers and oligomers. UVS 91 consisted ofmercapto-esters and tetrahydrofurfuryl methacrylate. The polyacrylatesubstrate consisted of ethoxylated bisphenol a-dimethacrylatebifunctional monomers and 1 wt % 2,2-dimethoxy-2-phenylacetophenonephotoinitiator. The contact angle represents the solid-liquid adhesionenergy per unit area. It was found the PRO-001 UV acrylate adhesive witha contact angle of 16±4° exhibited good adhesion with PEDOT:PSS. It wasfound that the PEDOT:PSS film was conductive on PRO-001 UV adhesivewithout sacrificing significant transparency. On the contrary, PEDOT:PSSdoes not adhere on hydrophobic substrates such as PDMS, and isnon-conductive on substrates such as PVA. Based on these observations,the acrylate-based UV adhesive, PRO-001 UV was selected as a printabletransparent substrate for subsequent experiments. However, had thedevice needed to be printed upon PDMS or PVA, an adhesive layer wouldhave been required.

The present invention also discloses an embodiment wherein thesemiconducting materials are printed by a method that involves firstproviding a syringe with a predetermined nozzle tip size. One skilled inthe art will recognize that the nozzle tip size will vary significantlybased on several factors, including but not limited to material flowcharacteristics, printing resolution required, print speed required, andthe pressure the inks will be under when printing. The ink comprisingthe semiconducting material is then loaded into the syringe, and thesyringe is placed under vacuum to prevent material from dripping fromthe syringe. When ready to print, the vacuum is turned off; the syringeis lowered until approximately when the ink at the nozzle tip touchesthe substrate. The amount of ink extruded was controlled by controllingthe hold-time in this position; generally this is done by holding thesyringe in place for a pre-determined period of time. The syringe isthen raised and placed under vacuum to prevent dripping.

The present invention also envisions a device containing multiple activeelectronic devices within it. Further disclosed is printing a conductivepattern connecting a first active electronic devices and a second activeelectronic device. An example of this is in wearable technology—a solarcell could be connected to a QD-LED. As illustrated in FIG. 2, in adevice 110 with multiple active electronic devices (120, 130, and 140),the connection between the active electronic devices may or may not bein a single plane—although FIG. 2 is a 2D drawing, one skilled in theart will recognize that the interconnect pattern can be in-plane (125),out-of-plane (135), an arbitrary direction (145), or any combinationthereof (for example, 155).

A specific use of the disclosed method involves making a quantum dotlight emitting diode (QD-LED) with a 3D printer. This method requiresprinting circular rings 20 connected to contact pads on a substrate 12using a conductive nanoparticle ink, as shown in FIG. 4A. One skilled inthe art will recognize that this method may be used with a wide range ofconductive nanoparticles, including, specifically, silver nanoparticles.The conductive nanoparticle ink is then annealed. As shown in FIG. 4B, aconductive polymer 310, such as PEDOT:PSS, is dispensed by nozzle 234 atapproximately the center of the printed circular ring 20. Thisdispensing continues until, as shown in FIG. 4C, the contact line 320touches the printed circular ring 20. The substrate is then heated. Asolution comprising relatively low concentrations of a polymer, andspecifically between about 0.05 and about 1.00 wt % of a hole-transportlayer material, such as poly-TPD, in an orthogonal solvent, such aschlorobenzene, is then created. That solution is added to a syringe anddispensed, after which the material is annealed. QDs, such as CdSe/ZnS,in a co-solvent mixture, such as dichlorobenzene/toluene, are thendispensed onto the annealed poly-TPD; the QDs are then dried. A liquidmetal, such as eutectic gallium indium (“EGaIn”), is then printed atapproximately the center of the printed circular ring, and a UV adhesiveis printed around the liquid metal to insulate it from the conductivenanoparticles the form the circular ring. The printed adhesive is thencured with a laser. A conductive silicone is then printed, and is thencured in an appropriate fashion. The final step in the disclosedembodiment is to print and cure a UV adhesive that encapsulates theprinted QD-LED device.

As shown in FIG. 5, this disclosed method may further involve printingconductive silicone as vertical interconnects (420) along with RTVsilicone (410) to connect the exposed liquid metal (80) to the contactpads (14), and to connect the anode (40) and cathode (80) of differentQD-LED (combination of 40, 50, 60, 80) layers (432, 434) so as to 3Dprint an array of QD-LEDs. Or, as discussed previously, it may involvescanning the surface using a 3D scanner to generate a geometricallyfaithful computer model of the surface, providing the computer model tothe 3D printer, and then adjusting the 3D printing process to enableconformal 3D printing on a substrate having a non-flat, 3D surface, oralternatively on a flat surface which has structural elements on thesurface, including but not limited to a rough or porous flat surface.

As one example of an embodiment of this invention, a regular glass slidewas cleaned with water, acetone and isopropanol for 15 minutes each.Circular rings of 1.5 mm radius connected to contact pads were thenprinted using a synthesized AgNP ink. The anode ring was printed with a3 cc syringe barrel through 33 ga nozzles (108 μm inner diameter) with abarrel pressure of 20 psi. The printer translation speed was maintainedat 0.5 mm/s, and the distance between the nozzle tip and the substratewas kept at 85 μm. The printed silver was annealed at 200° C. for 3hours. PEDOT:PSS (Sigma Aldrich, St. Louis, Mo.) was then dispensed atthe center of the ring until the contact line touched the ring. Thesubstrate was then heated to 150° C. for 15 minutes, which resulted in atransparent and conductive film. A 0.15 wt % poly-TPD (American DyeSource Inc, Quebec, Canada) solution in chlorobenzene was then dispensedand heated at 150° C. for 30 minutes. Subsequently, a 1 mg/ml ofCdSe/ZnS QD solution with either green (Ocean NanoTech, San Diego,Calif.) or orange-red (Ocean NanoTech, San Diego, Calif.) in aco-solvent mixture was then dispensed onto the annealed poly-TPD. Theprinted QD droplet was allowed to evaporate completely without heatingin a petri dish. Note that for printing of PEDOT:PSS, poly TPD and QDs,the ink was loaded into a 3 cc syringe barrel with a 32 ga nozzle. Thenozzle was registered to position at the center of anode ring. Ink wasprevented from dripping by applying a vacuum with a digital pressureregulator. During printing, the vacuum was turned off, the nozzle waslowered until the ink at the tip of the nozzle touched the substrate,and the position was held. The amount of ink extruded was controlled bycontrolling the hold-time, which corresponded in this example to anextrusion of approximately 0.8 μL. After printing, the nozzle was raisedand vacuum was resumed to prevent dripping of the ink. Upon completedrying of QDs, EGaIn (Sigma Aldrich, St. Louis, Mo.) was then printed atthe center of the ring with a digital pressure regulator to form aconformal liquid metal cathode. The 33 ga nozzle with a 3 cc barrelloaded with EGaIn was lowered to a gap of 100 μm from the surface ofprinted QD. A pressure of 4.5 psi was then applied for 3 seconds toprint a hemispherical liquid cathode to cover the quantum dots. UVadhesive (Novacentrix, Austin, Tex.) was then printed through 33 ganozzle, at a 50 μm gap with the printed anode, with a 5 mm/s translationspeed around EGaIn to insulate it from the AgNP anode ring. The printedadhesive was then cured with a 405 nm laser attached to the 3D printer(Fig. S2C). Conductive silicone (Silicone Solutions, Cuyahoga Falls,Ohio) was then printed in order to contact the exposed printed EGaInwith a pad before it was left overnight to vulcanize completely. UVadhesive (Novacentrix, Austin, Tex.) was printed using theaforementioned parameters and cured with the 405 nm laser to encapsulatethe printed QDLEDs.

The thicknesses of the individual layers was measured by profilometry tobe the following: AgNP silver ring, 80 μm; PEDOT:PSS, 200-300 nm;poly-TPD, 100-200 nm; quantum dots, 100-150 nm; EGaIn, 2-3 mm. TheQD-LED as fabricated was characterized with a source measure unit(Agilent, Santa Clara, Calif.), spectra were obtained with aspectrometer (Ocean Optics, Dunedin, Fla.), and the luminance wascalibrated with a luminance meter (Konica Minolta, Ramsey, N.J.). Allmeasurements were done under ambient conditions. The printed devicesexhibited pure color emission from the QD emissive layer with narrowfull-width at half-maxima (FWHM) of 43 nm for both colors. This is onpar with the color purity from QD-LEDs made with established methods.The color quality, based on a chromaticity diagram which maps the rangeof physically produced colors to an objective description of colorsensations by the average human. As indicated by the green (0.323,0.652) and orange-red (0.612, 0.383) Commission International del'Eclairage (CIE) 1931 chromaticity coordinates, the printed QD-LEDsgenerate highly saturated color emissions, which could enable thecreation of displays that can subtend the color gamut at levels greaterthan the high-definition television standard.

As an alternative embodiment of the present invention, synthesized AgNPswere printed on a spin-coated device, where the spin-coated layers aresimilar to a previously published all-solution processed fabricationmethod. Unlike the case for the EGaIn cathode, we found that AgNPsrequire an additional electron transport layer to achieveelectroluminescence. A 5 mg/ml solution of ZnO nanoparticles (SigmaAldrich, St. Louis, Mo.) was printed and then heated with a hotplate at150° C. for 30 minutes. The printed silver had a thickness of 80 μm,which was controlled by a combination of nozzle size (108 μm), printingspeed (0.5 mm/s), gap height from the substrate (80 μm) and appliedpressure. The ability to control the gap distance with a 3D printer is acritical factor in controlling the printing quality. If the gap is toosmall, excessive material would be squeezed out which would affect theplanar resolution and annealing time; in contrast, when the gap is toolarge, the extruded material would not be continuously drawn to thesubstrate. In addition, it was found that careful control of both theannealing condition and printing pattern are important in achievingsintering of the AgNPs without damaging the underlying layer. A slowtemperature ramping rate (10° C./min) and a spiral design were found tobe necessary to provide a sufficient surface-area-to-volume ratio forremoval of the solvent (water, ethylene glycol and polyacrylic acid) ata rate that causes insignificant damage to the underlying film. With aslow ramping rate and spiral design, a conductive cathode was formed.Once the parameter for cathode printing and sintering had beendetermined, the printing conditions for poly-TPD and PEDOT:PSS wereoptimized with a similar approach as described previously. The fullyprinted design consisted of a printed AgNP cathode and anode thatachieved a luminance of 11 cd/m² at 13 V. It should be noted that inthis example, for the desired purpose of the device, the performancewith the AgNP-based cathode was unsatisfactory by comparison with theEGaIn cathode for the following reasons. First, the printing of silverrequires heating of 200° C. for a minimum of 1 hour for its sinteringprocess, which damages the underlying layers when it is printed as acathode under ambient conditions. Second, although bulk silver has awork function of −4.3 eV, AgNPs have large surface-to-volume ratioswhich allows them to be easily oxidized to silver oxide, which has awork function of −5.0 eV

In another example, polyimide tape (McMaster-Carr, Robbinville, N.J.)was attached to a glass slide. Commercially available AgNP dispersed intetradecane (Sigma Aldrich, St. Louis, Mo.) was printed into a circularconductive ring (32 ga nozzle, 100 μm gap, 5 mm/s). The tape andconductive ring were then heated to 200° C. for an hour to achieve aresistivity of 2.7 μΩ·cm. The subsequent layer of QD-LED was thenprinted as described earlier, with modification of thesolvent-co-solvent ratio and QD concentrations. The QD-LED on thepolyimide tape was then peeled slowly from the glass slide andreattached to different substrates.

The present invention also discloses a 3-D Printer adapted for printingthese disclosed active electronic devices. As shown in FIG. 3, theprinter 210 comprises a plurality of material dispensing units 230, eachof which comprise a material storage location 232 and a nozzle 234. Thematerial storage location 232 includes any configuration where thematerial to be deposited is stored that can be accessed by the printer,and may include, but is not limited to, ink cartridges, filament spools,or syringe barrels. The material storage location 232 is then connectedto a nozzle 234, either directly, such as a syringe barrel 232 directlyconnected to the syringe nozzle 234 or a filament spool that feedsthermoplastic filaments directly to the nozzle, or indirectly, such asfrom an ink cartridge through tubes or hoses to the nozzle. The nozzle234 may have a predetermined nozzle tip size 236. The printer alsocomprises a printer stage 245 located under the plurality of nozzles,and a heating and cooling unit 240 for the printer stage 245. Utilizingpositive or negative pressure controlled by pressure regulation unit220, the nozzles deposit ink onto the stage, a substrate (247) on thestage, or similar, and the heating and cooling units 245 control thetemperature as the ink is being printed and as it is being cured orprocessed after the ink is deposited. To control where the printingoccurs, the entire stage, or part of the stage, may move in one or moredirections. Alternatively, or in combination with a moving stage, thenozzle or nozzles may move in one or more directions, independently oras a group. The entire stage, and possibly the stage and the syringes,may be enclosed by a cover or shield

While a single regular can control multiple nozzles, throughmultiplexing, other configurations may be desirable. For example, aseparate regulator may be provided for each nozzle, each regulatorcontrolling the pressure for a single nozzle. Alternatively, otherarrangements may be desirable, such as basing the number of regulatorson the level of control required by the viscosity of the materials to beprinted. For example, as part of the pressure regulation unit 220, theprinter may use multiple pressure regulators—a first pressure regulatorthat is connected to at least one syringe via tubes or hoses 225, and asecond pressure regulator that may be required to regulate thedeposition of inks having a viscosity within a predetermined viscosityrange.

The printer may also require a UV laser, typically for use in UV curingapplications. The printer may incorporate a vision system 250 focused onthe area on and around the stage 245. Additionally, the printer mayrequire a robot controller unit 260 for controlling various aspectsassociated with the printing process. This may involve connections(electric, wireless, or some other connection method) with associatedunits, including but not limited to the pressure regulator unit 220, thematerial dispensing units 230, the heating and/or cooling units 240, theplatform 245, or the vision system 250, or with a separate, stand alonecomputer.

As one example of this, an industrial robotic dispenser (Fisnar, Wayne,N.J.) was modified into a multi-head 3D printer, where up to fourdifferent inks can be loaded and independently controlled with anexternal I/O card and pressure regulators. The printer supports standardsize syringe barrels, and universal luer-lock needles. Tips from 27 to33 gauge (GA) have been used depending on the material viscosity andresolution required. For high precision printing, the barrel pressurewas regulated from 0 to 100 psi with a digital pressure regulator(Nordson Corporation, Westlake, Ohio). Vacuum suction control of theregulator was used to prevent dripping of low viscosity fluids. Higherviscosity inks were independently regulated with analog pressureregulators (Fisnar, Wayne, N.J.) for multi-material printing. Thedistance registration was calibrated with a digital CMOS laser sensor(Keyence, Itasca, Ill.), and the printer stage has a maximum resolutionof 1 μm per axis. Control of the 3D printer was achieved via customwritten LabVIEW programs through serial connection. Commerciallyavailable CAD software, Solidworks Premium 2014 (Dassault Systèmes,Vélizy-Villacoublay Cedex, France) was used for all modelingapplications. The typical printing strategy involved the formatting of3D CAD files into stereolithography (STL) format, followed by slicingthe model into G-code coordinates. The G-code was then translated to thecommand language of the robotic dispenser via a custom written LabVIEWprogram. In some instances, a Peltier stage heater was used and thetemperature was modulated with the applied voltage to optimize theprinting condition or for annealing the printed film. A UV laser (405nm) was also integrated to the printer arm to provide curing ofphotoactive materials, such as the UV adhesive.

Various modifications and variations of the invention in addition tothose shown and described herein will be apparent to those skilled inthe art without departing from the scope and spirit of the invention,and fall within the scope of the claims. Although the invention has beendescribed in connection with specific preferred embodiments, it shouldbe understood that the invention as claimed should not be unduly limitedto such specific embodiments.

What is claimed:
 1. A method of making a device, comprising the steps of: providing an ink comprising semiconductor particles; extruding the ink comprising semiconductor particles to form at least one active electronic layer via 3D printing; providing a conductive ink; and extruding the conductive ink to form at least one conductive pattern via 3D printing, wherein the at least one conductive pattern is adapted to allow an electric potential to be applied across the active electronic layer.
 2. The method according to claim 1, further comprising 3D printing at least one of an elastomeric matrix, organic polymers as charge transport layers, solid or liquid metal leads, nanoparticle semiconductors, or a UV-adhesive transparent substrate layer.
 3. The method according to claim 1, further comprising: identifying at least one material of an electrode, semiconductor, or polymer that possesses desired functionalities and exists in a printable format; and patterning of the at least one identified material via direct dispensing from a computer aided design (CAD)-designed construct onto a substrate.
 4. The method according to claim 3, further comprising: scanning the topology of the surface of the substrate; and providing information derived from the scanning step into the computer aided design (CAD) design of the device for conformal 3D printing.
 5. The method according to claim 4, wherein the substrate comprises a contact lens.
 6. The method according to claim 5, wherein electronics printed on the contact lenses provides a wearable display and/or a continuous on-eye glucose sensor.
 7. The method according to claim 4, wherein the substrate comprises at least one of a flat substrate, a non-flat substrate, a biological substrate, a glass substrate, a polyamide film, and a 3D printed substrate.
 8. The method according to claim 3, wherein the device is an active device from among quantum dot light-emitting diodes (QD-LEDs), MEMS devices, transistors, solar cells, piezoelectrics, batteries, fuel cells, and photodiodes.
 9. The method according to claim 3, incorporating other classes of nanoscale functional building blocks and devices, including metallic, semiconductor, plasmonic, biological, and ferroelectric materials.
 10. The method according to claim 3, wherein the active electronic layer is printed by a method comprising the steps of: providing a syringe with a predetermined nozzle tip size, loading the ink comprising the semiconducting particles into the syringe, placing the syringe under vacuum, turning off the vacuum, lowering the syringe until the ink at the nozzle tip touches the substrate, holding the syringe in place for a predetermined period of time, and raising the syringe and placing the syringe under vacuum.
 11. The method according to claim 3, further comprising printing a conductive pattern connecting the active electronic layer and a second active electronic layer.
 12. The method according to claim 3, further comprising dissolving or suspending the at least one material in a composition comprising an orthogonal solvent.
 13. The method according to claim 12, wherein the composition further comprises a second orthogonal solvent.
 14. The method according to claim 12, wherein the at least one material is present in the composition at between about 0.02 wt % and about 0.20 wt %.
 15. A method of making a quantum dot light emitting diode (QD-LED) with a 3D printer, comprising: printing circular rings connected to contact pads on a substrate using a conductive nanoparticle ink; annealing the printed conductive nanoparticles; dispensing a conductive polymer at approximately the center of the printed circular ring until a contact line defining the outer perimeter of the conductive polymer touches the printed circular ring; heating the substrate; dispensing and annealing a solution comprising between about 0.05 and about 0.20 wt % Poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidine (poly-TPD) in chlorobenzene; dispensing CdSe/ZnS QDs in a co-solvent mixture onto the annealed poly-TPD; drying the QDs; printing a liquid metal at approximately the center of the printed circular ring; printing an ultraviolet (UV) adhesive around the liquid metal to insulate it from the conductive nanoparticle ink; curing the printed adhesive with a laser; printing conductive silicone; and printing and curing UV adhesive to encapsulate the printed QD-LED.
 16. The method according to claim 15, further comprising: printing conductive silicone as vertical interconnects along with room temperature vulcanization (RTV) silicone to connect the exposed liquid metal to the contact pads and to connect the anode and cathode of different QD-LED layers so as to 3D print an array of QD-LEDs.
 17. The method according to claim 15, further comprising: scanning the surface using a 3D scanner to generate a geometrically faithful computer model of the surface; providing the computer model to the 3D printer, and adjusting the 3D printing process to enable conformal 3D printing on a substrate having a non-flat, 3D surface or a flat surface which has structural elements on the surface.
 18. The method according to claim 15, wherein the conductive nanoparticle is comprised of silver, the conductive polymer is poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and the liquid metal is EGaIn. 